Hydration for Distance Runners

Dehydration (a lack of fluids in your system) can pose real problems for completing a marathon. It causes reduced physical capacity and heat tolerance, deterioration in cognitive function, failure of thermoregulation, physical incapacity, and, ultimately, organ failure. Even repeated mild dehydration can cause renal stones, urinary tract infections, severe constipation, rectal afflictions and cutaneous membrane drying.

A lack of water in the system thickens blood in arteries and veins, reducing circulation, and along with that, reducing the oxygen and nutrients delivered to muscles, and also, the elimination of lactic acid from those muscles. As little as 2% dehydration can have measurable effects on performance, with up to a 7% decrease in endurance. Exercise is simply not possible with 10+% dehydration.

During a marathon, it is clearly possible to become dehydrated to these levels. Recorded measurements from elite runners show that they can lose up to two gallons of fluids, which would constitute at least 10 percent of the fluid base of a 180 lb. person. Far lesser amounts can still fall between 2 and 10% of a much smaller person.

You can estimate your fluid loss during a run by weighing yourself before and after that run, and dividing the pounds lost by 8.35 to convert that to gallons. Not all of that weight loss is lost fluids – some of that loss is the weight of stored glycogen in your muscle cells. But most of it is fluid loss, and for most of us, any loss of more than 2 or 3 lbs. during the course of a run is cause for concern.

We all know that heat dries out the human body. But the real culprit is humidity. For it is not the act of sweating that cools the human body, rather it is the *evaporation* of sweat that cools. Yet, in highly humid air, sweat cannot evaporate. The body sweats, and sweats, and sweats, all to little or no effect. The result is an overheated *and* dehydrated runner.

[The theory underlying performance fabrics in running attire is that by wicking away sweat to a broader area of the fabric, it increases the evaporative surface, thus raising the actual quantity of sweat evaporation, and producing a greater cooling effect.]

Other factors, such as diet, medications and menstrual status can affect overall hydration. The worst cases usually occur when the factors above all combine. You should learn the symptoms of dehydration, so you can help yourself and others out on the trail.

Thirst (or lack thereof), by itself, is a very poor indication of hydration levels. By itself, thirst indicates a dry mouth or throat. Rather, you should look for symptoms in combination: increased thirst, decreased urine output; concentrated urine and/or deeply yellow or amber in color, dry mouth and swollen tongue, the inability to sweat, weakness, confusion, dizziness, sluggishness, even fainting or heart palpitations (feeling that the heart is jumping or pounding).

Mild Dehydration can be treated by drinking frequent small amounts of room-temperature fluids, shade and rest. But for moderate to severe dehydration (anything involving dizziness or mental confusion, or more severe symptoms), seek medical attention for the afflicted runner. For those cases, treatment may require IV fluids and/or hospitalization.

Moderate to severe dehydration may be combined with other conditions requiring medical attention, e.g. Heat Stroke. When in doubt, get the runner off the trail and onto the ground, and call for medical assistance.

One of the reasons that medical attention is required for more serious symptoms, is that those can also be symptoms for conditions other than dehydration: Hyponatremia, Heat stroke/heat exhaustion, Hypoglycemia (“The Wall”) and Postural Hypotension (“Finish Line Syndrome”). Hypoglycemia is best discussed in other articles, as is heat stroke/exhaustion (except to note that when thermoregulation fails due to dehydration, they are far more likely).

[Procedural Hypotension (“PH”) is simply the phenomenon that when a runner stops running after a long, exerted effort, blood may pool in the legs and lower extremities, causing dizziness or fainting. While running, the action of leg muscles returns blood to the heart, but if that leg action suddenly stops, the heart is still sending blood down to the extremities at a high rate for several minutes thereafter. PH is the reason most races now force runners to keep moving after the finish line – a period of walking can help return blood from the legs back to the heart, as the heart rate slowly decelerates back to a resting normal.]

 Of all of conditions with symptoms similar to dehydration, Hyponatremia is most directly related to Hydration. Sometimes called EAH (Exercise-associated Hyponatremia) or the more exotic “water intoxication,” Hyponatremia is the direct opposite of Dehydration. It is the over-ingestion of water, which dangerously or fatally dilutes the electrolytes which the human body needs to function. Without electrolytes, human and animal cells cannot regulate their fluid content, and will eventually burst due to the pressure of excess internal fluids, and die.

[The required electrolytes include sodium (Na+), potassium (K+), chloride (Cl-), calcium (Ca2+), magnesium (Mg2+), bicarbonate (HCO3-), phosphate (PO42-), and sulfate (SO42-). Of these, the most important are sodium (which the body uses directly to regulate the fluid content of cells) and potassium (which the body uses to retain sodium).]

Hyponatremia most often results under exercise of long duration (such as a marathon), when the athlete sweats out electrolytes along with fluids, but replaces sweat only with plain water (and no food). It may also be worsened by some medications. Short of life-threatening complications, a deficit of electrolytes (like dehydration) can impact athletic performance, limit speed and endurance, and produce gastric distress, and can make the marathon a very uncomfortable experience.

The symptoms of Hyponatremia include fatigue, lightheadedness, weakness, cramping, weight gain, nausea, bloating and/or swelling, dizziness, headache, confusion, fainting, disorientation, seizures (severe cases) and coma (severe cases). Notice how many of those symptoms – especially the more serious ones – mimic those of dehydration, the direct opposite of Hyponatremia. Treating a hyponatremic runner for dehydration - pumping him or her with *additional* fluids - could be fatal!

For that reason alone, if you suspect Hyponatremia, or see any of the serous symptoms, obtain medical assistance for the afflicted runner.

The usual clinical treatment for Hyponatremia is fluid restriction and diuretic therapy. At the end of marathons, you will often find medical personnel addressing the runners, asking them questions to gauge their mental capacity. Runners who are confused, dizzy, or unresponsive are taken in for further diagnosis – dehydration, hyponatremia, heat stroke, or some other condition. But even these trained medical persons cannot pinpoint the problem without further testing.

Neither should you. If a runner beside you appears dizzy, or faint, or confused, or utterly lethargic, please get medical assistance for that runner.

Because it is hard to spot and diagnose, Hyponatremia scares many race directors. But compared to dehydration, it is a rare occurrence. The reason is that serious Hyponatremia requires the perfect storm of causes, a combination of factors unlikely to occur to anyone paying attention to diet and hydration before and during an athletic event. The American diet is so over-loaded with sodium, that most Americans have way too much of it in their systems, to begin with. [It is ironic that electrolyte beverages such as Gatorade are still labeled “low sodium” by the U.S. Food and Drug Administration.] Conversely, a very “healthy” diet, one that reduces sodium, may paradoxically place a runner at a greater disk for hyponatremic distress.

Hyponatremia is the raison d’ete of most sports beverages, starting with Gatorade, and its competitors. It is important to remember, however, that even those products do not fully replace the amount of electrolytes lost from sweat during intense exercise in warm and/or humid weather.   Base Gatorade contains 120 millgram of Sodium per serving; the "endurance formula" contains 200 millgrams.  But if you finish a training run or a race covered in salt crystals (left behind when the liquid part of your sweat evaporated) guess what?    You're losing more sodium, and faster, than Gatorade or any other sports drink can put back in you.

But if you just drink a sports beverage in lieu of plain water while running, and consume at least some nutrition along the way, your chances of suffering from Hyponatremia are still quite low.  You're going to lose sodium during a distance run, but not enough to reach critical levels.  You are still far more likely to suffer from Dehydration.

Proper hydration for the marathon then, is a BALANCE – neither too many fluids, nor too few. If you lose fluids (and electrolytes), you must either replace them fully, or start the race with enough surplus to spare.

It is highly unlikely that you can drink and absorb as many fluids during a marathon as you will lose from sweat, particularly on a hot or humid day. If your sweat losses are a gallon or more of fluids, it is unlikely that you can consume that many fluids on the race course. The average fluid cup used at aid stations during footraces contains only 4 ounces of fluids, so you would need to drink at least 16 of them to match one gallon of fluids lost.

However, your marathon may not have enough aid stations to give you 16 or more cups of fluids. Or you may be uncomfortable drinking a full cup (or multiple cups) at every aid station. You may be even more uncomfortable running with a stomach full of water. [Excess fluids "sloshing around" in the stomach can cause stomach ache, or "Montezuma’s Revenge."]

In that case (or in the case of training runs, which offer fewer fluids on the course), you need to bring your fluids with you. Whether from a hand-held bottle, one or more small or large bottles strapped to a belt, or a full hydration backpack, the coaches of the MCC Summer Training Program will require you to carry fluids (i.e. a sports beverage) with you on every Saturday training run. No fluids, no run.

But even if you could drink as much as you lose while running, it is unlikely that you could digest it. The average maximum digestion of fluids from the stomach is about 1 liter per hour. Yet, actual digestion is proportionately related to the amount of fluids (vs. solids) in the stomach. Where the stomach is full of fluids (sloshing around in a way which is uncomfortable to most athletes), digestion is the greatest. But half full, and digestion is only half as much. A few ounces of fluid, and there is barely any digestion at all.

And during extended exercise, digestion of food and fluids plummets. Under the duress of marathon running, the body diverts bloodflow from the digestive tract to working muscles, and to the skin, for thermoregulation. Your actual digestion of fluids, particularly at the latter stages of the race, when your stomach is often queasy, is far less than the optimum one liter per hour.

How then, to counter the great loss of fluids while exercising for hours in hot or humid conditions?

In addition to taking in fluids during the run, you must also go *into* the race or training run well-hydrated. You must begin a race (or any long training run, for that matter), on the high end of well-hydrated, almost over-hydrated.

This requires a “hydra-loading” similar to the traditional “carbo-loading” prior to a marathon. During the days before the event, you should ingest extra fluids in small doses throughout the day. A few ounces extra every hour will add up to 32 ounces over an entire day. Three days of this will equal a gallon and a half of extra fluids. Because the human body will urinate away about 2/3 of extra fluids consumed, you will be visiting the bathroom a lot if you "hydraload." But you _will_ retain that 1/3 extra, your margin for error during the race.

As you are pre-hydrating, remember to watch your electrolytes. Sports beverages are better than plain water, and salty foods are a great way not only to get needed sodium into your system, but they can also trigger the extra fluid intake you need.

In the days before a long run or a race, watch your carbo-loading, so that extra carbs in your stomach don’t eliminate your ability to hydrate. Clear-colored urine is a sign prior to the race, than you are on your way to proper hydration.

And on race day, get to the starting line early, so you will have time for the inevitable bathroom break (and any line associated with it) well prior to the start of the race.

Above all, practice good hydration habits prior to, and during your training runs leading up to race day. Like most other facets of the marathon, hydration is not something you can ignore during training, and then change on race day. Before every training run, watch your hydration the day and evening prior to the run, and then practice carrying the right fluids with you on the run and consuming them at frequent intervals.

Proper hydration for the marathon is a balance. Learn the proper balance for your own physiology during your training runs, and then not only will it be second nature during the marathon, but you will feel a heck of a lot better than most on race day.

Copyright 2007-2011 by John H. Steitz. Please contact the author for references and data researched for this article.